Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 17)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
FIGURE 5-1. Components of the OBS-3+ (left) and orientation source
beam and detector acceptance cone of the OBS-3+ (top right) and
OBS300 (bottom right)
The beam divergence angle of the VCSEL source is 42 o (95% of the beam
power is contained within a 42 o cone). The sensor can see to a distance of
approximately 50 cm (20 in) in very clean water.
5.4 Measurement Details
The OBS-3+ and OBS300 are dual range sensors having both a low- and a
high-range output. The standard low ranges are 250, 500, 1000, or 4000 NTU,
and the corresponding high ranges are 1000, 2000, and 4000 NTU. OBS
sensors can be purchased with a 4 to 20 mA current output on the low range
and a 0 to 5 V output on the high range. Voltage outputs can be 0 to 2.5 or 0 to
5 V; see Section 6, Specifications. It is also possible to purchase sensors
configured to operate from 5-V power, however, the output span is limited to
2.5 V.
The sensor needs to be connected to a datalogger, current meter, or CTD
instrument. The datalogger (or other device) powers the sensor, digitizes its
analog signals, computes NTU and Suspended Solids Concentration (SSC)
values, and records the statistical result in flash memory. To make the
conversion from digitized signals to engineering units (for example, NTU, mg
L –1 , and ppm), the datalogger must have the calibration equations in its
operating program or the conversion must be done in post processing.
When using some current meters or CTD instruments, the OBS
sensor should be calibrated while it is connected to the device
exactly as it will be used. The reason for this is that the factory
calibration is performed with a NIST-traceable digital multimeter
and the numerical values reported by some host devices not NIST-
certified will be different.
NOTE
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